The present invention relates to a rotary screw compressor having at least one stage and each stage being provided with at least one relief lift valve.
It is common to provide rotary screw air compressors with devices for unloading the compressor at starting up and at periods with low demand on compressed air. Therethrough the power consumption can be reduced. A frequently used method is to throttle the compressor inlet channel down to about 10% of the normal inlet pressure. Although a certain amount of drive energy can be saved this way there still remains a considerable work to compress the air. Another solution is to provide the wall of the working space with valve controlled openings for reducing the capacity and the internal volume ratio (V.sub.i) of the compressor during the starting up periods as disclosed in GB-1 576 230. This disclosure, however, does not mention anything about the valve arrangement for the openings. Normally such openings are provided with so called lift valves as disclosed in U.S. Pat. No. 4,453,900. The lift valves in U.S. Pat. No. 4,453,900 are, as is the common way, actuated by oil pressure, which is relatively complicated, requiring circumstantial measures for manufacturing and maintenance.